GPS is Great, But...

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GPS is Great, But...
By Peter F. Prowant

The Navstar Global Positioning System (commonly called GPS), provides all mariners, whether the recreational boater or the professional mariner the ability to navigate their vessels with much greater accuracy than previously possible. GPS receivers, from the widely available and inexpensive hand held units to the most sophisticated chart plotter units, make navigation, particularly for the neophyte, much simpler and quicker. No one disagrees that GPS represents a quantum leap in navigational tools available to the mariner. The increased reliance on GPS, however, leads to complacency in learning other methods of navigation: plotting by relative bearings, dead reckoning and celestial navigation.

Complete reliance on GPS in lieu of other navigation methods is a mistake. The inherent, commonly misunderstood, belief is that GPS receivers capable of accuracy within several meters, place the mariner exactly on the plotted position of the chart or at the exact position of the electronic chart loaded into the chartplotter or navigation computer.

The accuracy of a position plotted directly from GPS is, however, limited to the accuracy of the chart. A good example of this is the recent observation by two Bluewater customers within a week, both using electronic chartplotters, that the GPS was not accurate in the vicinity of the island of Bimini, on the western side of the Bahaman Islands. Interestingly, the customers were using chartplotters and digitized cartography from different manufacturers. Yet the complaint and the position errors provided by the chartplotters were exactly the same, placing the vessels approximately 200 yards on the island itself, while navigating into the harbor. How can this be?

Simply, the position of Bimini on National Imagery and Mapping Agency Charts and British Admiralty Charts is based on a chart first published in 1844, when surveyed positions were based entirely on celestial observations, hence the apparent error. It is rather amazing to me, that charted positions based on such observations are so accurate! Logically, the question arises, why not just correct the position of Bimini? Unfortunately, correcting charted positions is not that simple, for to move the position of Bimini on the chart would mean that all charted positions of other islands, shoals, coral heads, entrances, lights and so forth on the chart would also have to be corrected relative to each other.

Bimini is but one and not an extreme example of the difference between a charted position and the position of an island indicated by a GPS receiver. Many people cruising the Pacific complain the "GPS is off by half a mile in the Pacific." Many of the charts for the world, other than those areas with heavy commercial traffic, are based on surveys from the beginning of the nineteenth century.

The inherent problem of correcting charted positions lies in the fact that the earth is not perfectly round. It is higher in some areas than others, flatter in some locations and depressed in still others. Charts are compiled and drawn with this factored in. All charts are drawn to mathematically calculated triangulation networks called datums. The datum is calculated from an origin point. Examples include the Tokyo Datum which has its origin in Tokyo; the Indian Datum with its origin in Kalianpur, central India. The North American Datum, 1927 (NAD 27) has been used in the United States for about 50 years and is based on the latitude and longitude of a triangulation station (the reference point) at Mead’s Ranch in Kansas. NAD 27 it is being replaced by datums based the World Geodetic System (WGS 84), which is also the default datum referenced by GPS receivers.

Boaters familiar with the Intracoastal Waterway in the United States know that GPS receivers are simply no substitute for good piloting skills. The position of the vessel is determined by the identification of channel markers, landmarks, use of ranges and most importantly, watching the channel itself, particularly in the salt marshes of the southeastern United States. I have seen channel markers in the Intracoastal Waterway as high and dry as a golf course, with the channel shifted more than fifty yards, from its charted position. By far, the most important pieces of navigational equipment are binoculars and a hand-bearing compass.

Unfortunately, recreational cruisers usually give little thought to learning celestial navigation, even those that circumnavigate. After all, getting ones position, in less than a minute by pressing a button, is much easier. Yet, I believe that anyone venturing offshore for any length of time should learn basic celestial navigation for a non-electronic based back-up system. Celestial navigation is not particularly difficult, although practice is necessary to garner proficiency and accuracy. Celestial navigation also demands that the mariner set up a disciplined routine in keeping a dead reckoning plot, planning the times of sights and fixing the position, then keeping a log. Keep in mind that both the United States Coast Guard and the Maritime and Coastguard Agency of the United Kingdom require proficiency in celestial navigation for offshore captain’s licenses.

Finally, it seems remarkable to me that so few recreational boaters bother with swinging their compass and creating a deviation card, a task easily completed in half a day, although hardly a boat is delivered without a compass. Chapman’s Piloting: Seamanship and Small Boat Handling by Elbert S. Maloney has an excellent chapter on compass adjustment. Some of the most satisfying passage making I have done, almost twenty years ago, was sailing from island to island in the Caribbean with a compass and chart. Never did I miss a port.

Today, GPS is the primary navigational aid used by recreational boaters, naval and commercial ships. Total reliance on it, though, can put your vessel and possibly your life, at risk. Indeed, the satisfaction of accomplishing a journey of steering by compass and fixing a position by the sun is one of the best I can think of.